1. Field of the Invention
This invention is in the field of calcining fine grained materials such as cement meal wherein sulfur compounds and sulfur dioxide are monitored and reacted with sulfur bonding substances in predetermined amounts to produce an exhaust gas which is relatively free of sulfur compounds and solids which contain the reaction products.
2. Description of the Prior Art
The formation of injurious material concentrations of sulfur compounds in the circulation of products through various material conversion systems is frequently a severe problem. For example, the concentration of alkali or sulfur compounds occur quite frequently in calcining installations for the manufacture of cement and often lead to the formation of injurious incrustations.
Sulfur, for example, occurs in sulfidic or sulfate compounds of the raw meal which contains calcium carbonate, the raw meal being calcined into clinker in the normal process to use of suspension type gas preheaters and rotary tubular kilns. As a result of the temperatures occurring in the system, which may be around 1400.degree. C. in the clinker zone, sulfur compounds are dissociated while liberating SO.sub.2 into the process gas. Further amounts of SO.sub.2 may be introduced into the system through the use of sulfurous fuels.
Sulfur dioxide produced in the hot gases of the calcining system reacts with the alkali and alkaline earth compounds contained in the raw meal to form alkali sulfates. For example, potassium oxide resulting from the presence of silicate compounds of the minerals of the raw meal reacts with sulfur dioxide according to the equation: EQU K.sub.2 O+SO.sub.2 +1/2O.sub.2 =K.sub.2 SO.sub.4
Since alkali metal sulfates have low vapor pressures, these sulfates are discharged from the calcining system together with the clinker.
A portion of the sulfur dioxide present reacts with lime components which have been deacidified from the calcium carbonate form to calcium oxide and thereby form calcium sulfate or gypsum. At temperatures less than about 1000.degree. C., the equilibrium tends to the formation of calcium sulfate and calcium sulfide. At temperatures in excess of about 1000.degree. C., the reaction tends to the formation of calcium oxide and sulfur dioxide. The overall reaction can be represented as follows: EQU 4CaO+4SO.sub.2 .revreaction.3CaSO.sub.4 +CaS
Calcium sulfate, however, is not stable at temperatures above 1000.degree. C. but dissociates in the clinker zone at approximately 1400.degree. C. to form CaO and SO.sub.2. The sulfur dioxide thereby liberated appears in the gases of the calcining system and forms the primary cause for the formation of sulfur circulations (Walter H. Duda, Zement-Data-Book, 2nd Edition, 1977, Bauverlag GmbH, Wiesbaden and Berlin; McDonald and Evans, London, Page 6, 1.4.3: Schwefel; Reprint from the periodical "Zement-Kalk-Gips" (ZKG), Bauverlag GmbH, Wiesbaden, 1960, pages 36 through 44: Weber "Warmevorgange im Drehofen unter Berucksichtigung der Kreislaufvorgange und Phasenbildung").
A balanced sulfur concentration within a calcining system wherein the intake of sulfur from the raw meal and fuel as well as loss in the clinker zone, dust, and allowable concentrations of exhaust gas are in a state of equilibrium can be achieved only with great difficulty, in view of the multitude of influences on the raw material side and because of the variable reaction determining parameters. The formation of internal dust circulation also plays a part since the meal circulation can, for example, achieve 1.5 to 2 times the amount of delivered meal in the lowest heat exchanger stage.
Under some conditions, dust and material circulations can interact when the original flow is disrupted due to incrustations or lining damage, particularly in the meal intake portion and as a result, displacements in the intended temperature zones also occur.
As a result, continued circulation can lead to extremely high built-up concentrations of compounds at individual locations of the calcining system even though these materials are present in only small amounts in the raw material or in the fuel.
In the case of sulfur, circulation of appreciable concentrations causes undesired incrustation formations, caused by eutectics in the range from 800.degree. to 1000.degree. C. within a suspension type heat exchanger. This reduces the output of the system and may possibly lead to operating disruptions. Thus, when the concentration of SO.sub.2 which cannot be reduced to a sufficient degree by means of sulfate formation appears in the gas stream of the suspension type heat exchanger, the SO.sub.2 emission of the exhaust gas of the installation may increase far above the allowable limits of, for example, 200 to 250 ppm to 1000 to 1500 ppm.